Silica gels have numerous industrial applications, including use as sorbents and as catalyst supports, including supports for olefin polymerization catalysts. Specifically, these olefin polymerization catalysts include a catalytic transition metal, such as chromium, deposited on a silica xerogel support that may be activated by high-temperature oxidation. Olefins are polymerized in the presence of such catalysts to produce various polyolefins having different molecular weight distributions and melt indices, depending upon the particular temperature, pressure, solvent, catalyst and other polymerization conditions employed.
The production of low molecular weight, high melt index polyolefins is of particular interest because such polyolefins are widely used in films and sheets, in extrusion coating, in injection and rotational molding, and in similar industrial applications. In many applications, such as in extrusion or molding applications, a polyolefin, such as polyethylene, having a broad or multimodal (e.g. bimodal) molecular weight distribution exhibits excellent processing characteristics, such as a faster throughput rate with lower energy requirements.
In many applications, polyolefin toughness, strength, and environmental stress cracking resistance (ESCR) are important properties. These properties are enhanced when the polyolefin has a high molecular weight. However, as the molecular weight of the polyolefin increases, the ability to process the polyolefin resin usually decreases. Therefore, by providing a polyolefin having a broad or bimodal molecular weight distribution, the important physical properties exhibited by high molecular weight resins are retained, and advantageous processing properties, particularly extrudability, of the polyolefin resin are improved.
When a polyolefin has a bimodal molecular weight distribution, a molecular weight distribution plot (by size exclusion chromatography) of concentration of species of specific molecular weight vs. log molecular weight has at least two maxima. The two maxima are characteristic of a bimodal molecular weight distribution, and the maxima need not be equivalent in magnitude or widely separated for the resin to exhibit the properties of a bimodal polyolefin.
Three major techniques have been proposed or used to produce polyolefin resins having a broad or bimodal molecular weight distribution. One technique is post reactor or melt blending of two or more polyolefins having different molecular weights. This technique has the disadvantages of requiring complete homogenization of at least two polyolefin resins and an attendant high cost. A second technique utilizes multistage reactors, but this technique has a low efficiency and, again, an attendant high cost. The third, and most desirable, technique is the direct production of a broad or bimodal polyolefin from a single catalyst, or catalyst mixture, in a single reactor. Such a process would provide a polyolefin having a broad or bimodal molecular weight distribution in situ, with the polyolefin components having a different molecular weight being intimately mixed on the subparticle level.
The production of a broad or bimodal polyethylene resin by a single catalyst, or by a catalyst mixture, in a single reactor has been disclosed. For example, U.S. Pat. No. 4,025,707 discloses the preparation of ethylene homopolymers and copolymers of broadened molecular weight distribution by utilizing a mixed catalyst comprising several portions of the same or different chromium components, and metal promoted variations thereof, wherein each portion is activated at a different temperature. U.S. Pat. No. 4,560,733 discloses combining magnesium and titanium-containing catalyst components. The catalyst is prepared by milling a blend of at least two silica-containing components having different melt index potentials. U.S. Pat. No. 4,918,038 discloses a mixed catalyst system, based on vanadium, to control the molecular weight distribution of the polyolefin.
Although such techniques have improved the processing characteristics of the polyolefin, the processing advantage has been largely offset by a corresponding decrease in one or more essential physical properties of the polyolefin. For example, the polyolefin resins obtained in accordance with U.S. Pat. No. 4,025,707 have good die swell characteristics, acceptable environmental stress cracking resistance (ESCR) and flow properties, but polymer densities are too low to provide the necessary stiffness for blown bottles. In addition, polyolefin produced in accordance with the catalysts of U.S. Pat. No. 4,560,733 have sufficiently high densities (0.960 and greater), but typically are deficient in their resistance to environmental stress cracking.
A catalyst system that utilized a blended support, wherein different grades of a particular type of support, such as silica or alumina, having a different average pore radius are blended, have failed to provide a polyolefin having a broad or bimodal molecular weight distribution. Theoretically, a catalyst prepared on such a blended support should provide a broad or bimodal molecular weight polyolefin because the differing average pore radii of the different support grades provide polyolefins of a different molecular weight. However, because such a blended support failed to provide a bimodal polyolefin, investigators have continued to seek a catalyst support that exhibits a bimodal pore radius distribution and that, when used as a catalyst support, provides a polyolefin with a broad or bimodal molecular weight distribution.
Several patents disclose that preparation of a bimodal alumina particle, or a bimodal silica-alumina particle. For example, Murata in U.S. Pat. No. 3,949,030 discloses a cellular fused silica having a bimodal closed cell structure produced from a mixture of silica and boron oxynitride. Heating the silica. boron oxynitride mixture to the melting point of the silica decomposes the boron oxynitride, thereby releasing a gas that creates cells in the silica and providing a silica matrix that exhibits a bimodal pore diameter. Several other patents, for example, Leach U.S. Pat. No. 3,898,322; Kim et al. U.S. Patent Nos. 4,257,922 and 4,294,685; and Bouge et al. U.S. Patent No. 4,315,839, each disclose an alumina that exhibits bimodal pore diameter distributions. Such bimodal aluminas are useful as components of hydroprocessing catalysts.
R. Snel, in a series of four publications in Applied Catalysts, 11 p. 271-280 (1984); 12 p. 189-200 (1984); 12 p. 347-357 (1984) and 33 p. 281-294 (1987), disclosed silica-alumina gels having a bimodal pore structure. Snel also disclosed the use of a nitrogen base as a pore regulating agent. The bimodal silica gel particle of the invention, on the other hand, is essentially free of alumina (e.g. includes less than about 5%, and preferably less than about 2%, alumina), does not include a pore regulating agent, and is prepared by a method that is substantially different from the method employed by R. Snel.
As previously stated, it would be highly desirable to produce a bimodal polyolefin directly in a single reactor, i.e., without the need to blend polyolefins having different molecular weight distributions in order to obtain the advantages of a bimodal polyolefin. It is even more highly desirable to provide a high activity polymerization catalyst that produces high quality polyolefins having a broad or bimodal molecular weight distribution. To achieve this goal, it would be desirable to provide a silica catalyst support wherein each silica xerogel particle exhibits a bimodal pore radius distribution. The present invention provides a silica xerogel particle that exhibits a bimodal pore radius distribution and provides catalysts that yield polyolefins having a broad or bimodal molecular weight distribution.
Therefore, the invention is directed to a silica gel particle that exhibits a bimodal pore radius distribution, and that is used as a support for a catalyst that yields polyolefins having broad or bimodal molecular weight distributions. The silica xerogel support is prepared from a silica hydrogel prepared by the method of the present invention. The silica hydrogel is prepared by first and second precipitations of the silica hydrogel from an aqueous solution, at two distinct pH values, to provide a silica hydrogel particle that exhibits a bimodal pore radius distribution. The difference in pore radius between a first average pore radius and a second average pore radius in the same silica xerogel particle is at least about 20 .ANG. (angstroms), thereby demonstrating the bimodal properties of the silica hydrogel particle. Converting the silica hydrogel particle to a silica xerogel particle does not destroy the bimodal pore radius distribution of the silica gel particle.
In general, the bimodal silica gel of the invention is prepared by adding an aqueous silicate solution to an aqueous acid solution to precipitate a silica hydrogel in an acidic medium. After an aging step, an additional amount of the silicate solution is added to the aqueous mixture including the silica hydrogel until the pH of the aqueous mixture is raised to at least about 9. Then, an acid solution is added to the mixture having a pH of at least about 9 to precipitate additional silicate on the silica hydrogel in an alkaline medium. The average pore radius of the silica hydrogel precipitated in the first precipitation is different from the average pore radius of the silica hydrogel precipitated in the second precipitation. Accordingly, a silica hydrogel particle that has a bimodal pore radius distribution within a single particle, and that is essentially free of alumina, is provided. Upon further processing, the bimodal silica hydrogel is converted into a bimodal silica xerogel. The resulting bimodal silica xerogel then is useful as a support for a polymerization catalyst. The polymerization catalyst, such as a chromium-containing catalyst, provides a polyolefin that demonstrates a broad or bimodal molecular weight distribution.
The precipitation of a silica hydrogel by adding an aqueous silicate solution to an aqueous acid solution is disclosed in Stoewener U.S. Pat. No. 1,738,315. Stoewener discloses adding the silicate solution to a sufficient amount of acid solution to neutralize the silicate. Stoewener does not teach or suggest a second precipitation of the silicate, and does not teach or suggest controlling the pH in the precipitation steps to provide a bimodal silica hydrogel. Nozemack et al., in U.S. Pat. No. 4,780,446, disclose an alumina-silica cogel that includes from about 91.5 to about 94.5 percent by weight alumina and that has a wide pore size distribution. The cogel of Nozemack et al. is prepared by adding an aluminum sulfate solution, a sodium aluminate solution and sodium silicate solution to a water heel, and maintaining the pH of the mixture in the range of 7.6 to 8.4. After the addition of the ingredients to the water heel, additional sodium aluminate solution is added to the mixture to increase the pH of the mixture to 9.6 to 10.3. Nozemack et al. do not teach or suggest a second precipitation to provide a bimodal silica-alumina hydrogel, and Nozemack et al. do not teach or suggest a bimodal silica hydrogel particle that is essentially free of alumina, such as a bimodal silica hydrogel particle including less than about 5 wt. % alumina.
It is evident that a polyolefin exhibiting a broad or bimodal molecular weight distribution is desirable, or necessary, in several industrial applications. Attempts to provide such a polyolefin in a single reactor using a single catalyst, and thereby obviate the blending of polymers having a different molecular weight distribution, or of blending catalysts, has not been entirely successful. Therefore, the invention is directed to providing polyolefins having broad or bimodal molecular weight distributions from a single catalyst in a single reactor, wherein the catalyst support is a silica xerogel particle that is characterized by a bimodal pore radius distribution and is essentially free of alumina.